jpetersen
No. DC is a clear offset either up or down.
 
Let's eliminate things systematically one at a time, else we'll be guessing forever.
 
Please record some silence with your mic connected and the input level at what you normally use.
Do the same with the mic and cable disconnected.
 
If you get the low waveform only when the mic is connected, then it really is the mic.
Else it is your interface or some other source of interference.

great idea jpeterson, I will try this when I get home later.   I will post the results of both of these tests.  Thank you!!

Sanderxpander
Could it be DC offset of some sort? I've never used that checkbox.

I wondered about that and actually processed his original clip but there's no change to the 25Hz build up.
 
Op, if you crank your monitoring up REALLY loud, can you hear any subsonic rumbling anywhere?
Do it with your mic plugged in and the input gain at normal recording level - make sure input echo is on

Bristol_Jonesey

Sanderxpander
Could it be DC offset of some sort? I've never used that checkbox.

I wondered about that and actually processed his original clip but there's no change to the 25Hz build up.
 
Op, if you crank your monitoring up REALLY loud, can you hear any subsonic rumbling anywhere?
Do it with your mic plugged in and the input gain at normal recording level - make sure input echo is on

will test for that later tonight and post results.  thank you!

Eh DON'T crank up your monitoring if your highly sensitive condenser mic is in the same room!!

Sanderxpander
Eh DON'T crank up your monitoring if your highly sensitive condenser mic is in the same room!!

oh i planned on doing it with my high powered HD6 Mix Pro headphones which go down to like 8Hz. :)

Oh good haha. A feedback loop like that would NOT be fun.

By the way the 214 seems to have a low cut switch, have you tried to see what happens if you enable it?

Sanderxpander
By the way the 214 seems to have a low cut switch, have you tried to see what happens if you enable it?

Hi there, yea I didn't mention it because I already have it enabled (the manual says for lead vocals its best to have it enabled so it scoops at the low end rumble).

According to the user manual, the low-cut is down almost -30dB at 25Hz.
That means it isn't coming from the room and it's not rumble coming through the floor.
If it were, it'd be very loud.
 

So it's either the phantom power from the interface 
or, and most likely, the mic itself is faulty.